Greek investigators struggling to determine the cause of this week's deadly airplane crash have encountered an unexpected setback: One of the black boxes with vital flight data has not been recovered, and the other may be damaged.

A second problem in the search for clues in the mountains north of Athens lies in the design of the separate cockpit voice recorder, which stores only the last 30 minutes of audio. Because the Helios Airways flight apparently flew for hours after the pilots were disabled, the most valuable information may have been overwritten.

In a digital era where airplanes are jammed with telephones, satellite TV and Wi-Fi wireless links, relying on physical black boxes that can be lost or incinerated seems almost outmoded. So why don't commercial flights beam important data back to the ground every few microseconds?

Krishna Kavi, chairman of the computer science and engineering department at the University of North Texas, says that piggybacking an air-to-ground connection on top of an existing transmission link is feasible.

"You don't need a very expensive communication network to relay information from plane to ground," said Kavi, who has written about black box design. "If you can use a cell phone, recorders should be able to transmit that information to an airline company."

There are good reasons for air-to-ground streaming. If an in-flight recorder is lost or destroyed, the additional precaution of backing up flight data on the ground would help investigators reconstruct what happened.

An air-to-ground link also would permit information to be retained much longer; digital flight data recorders on current Boeing planes store data for 25 hours, and cockpit voice recorders store between 30 minutes and 2 hours. And searching for black boxes with damaged radio beacons is frustrating and expensive.

The federal government has never required air-to-ground data feeds, primarily because officials believe the cost of a reliable system remains too high.

"We feel our money's better spent making the recorders so they will survive, they can be found, and provide the economically reliable data that we need," said James Cash, chief of the vehicle recorder division at the National Transportation Safety Board. "I don't think you could ever make the economic justification that a real-time download is economically feasible to impose on the industry."

Other reasons include the challenge of engineering a system that could transmit even when the plane is in an unusual flight attitude--such as inverted or in a dive--instead of in normal flight. The NTSB says it's aware of only three crashes, not counting the Sept. 11, 2001 attacks, in which the flight data recorders were not recovered.

Really orange, not black
Dave Warren's interest in recording music led to the creation of the original black box in the mid-1950s.

An engineer at the Aeronautical Research Laboratory in Melbourne, Australia, Warren was helping to investigate the cause of an airplane crash that had no witnesses. He realized that reconstructing what happened would be easier if the information was stored onboard.

"Dave then recalled the world's first miniature (tape) recorder that he had recently seen at a trade fair," according to a history prepared by the Australian government. "Suddenly he could visualize such a recorder placed in all aircraft, continually recording details and able to be recovered after a crash."

Today's black boxes--a term that encompasses the flight data recorder and the cockpit voice recorder--are required on commercial airliners and regulated by the Federal Aviation Administration and the European Organization for Civil Aviation Equipment. To aid in recovery, both are painted orange.

They're designed to survive the fiery temperatures and intense pressures of a crash. Regulations require that the boxes be resistant to 5,000 pounds of pressure, temperatures of 2,000 degrees Fahrenheit, and deep sea pressure of 20,000 feet. (Similar black boxes are popping up in cars too.)

Flight data recorders are plugged into the aircraft's main data bus, which lets them record up to 88 different forms of information such as altitude, airspeed, and the position of the stick used by pilots to control the plane's elevator and ailerons.

The data they safeguard can yield explanations that can help prevent future crashes. A January 2000 NTSB report on an Alaska Airlines flight that crashed off the coast of California determined, largely based on black box data, that the horizontal stabilizer on the tail was

About the author

Declan McCullagh is the chief political correspondent for CNET. You can e-mail him or follow him on Twitter as declanm. Declan previously was a reporter for Time and the Washington bureau chief for Wired and wrote the Taking Liberties section and Other People's Money column for CBS News' Web site.
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